The correct answer is: b) Hunting and gathering activities.
Prehistoric stone tools are a type of archaeological artifact that can be used to infer the activities of early humans. The discovery of prehistoric stone tools in Sikkim indicates that early humans in this region were engaged in hunting and gathering activities. This is because stone tools are often used for hunting and gathering, and the presence of stone tools in a particular area suggests that early humans were using that area for these activities.
The other options are incorrect because they do not provide a clear explanation for the discovery of prehistoric stone tools in Sikkim. Option a) is incorrect because there is no evidence to suggest that early humans in Sikkim were engaged in advanced agricultural practices. Option c) is incorrect because there is no evidence to suggest that early humans in Sikkim were living in large-scale settlements. Option d) is incorrect because there is no evidence to suggest that early humans in Sikkim were engaging in early religious rituals.
In conclusion, the discovery of prehistoric stone tools in Sikkim primarily indicates that early humans in this region were engaged in hunting and gathering activities.